Neisseria meningitidis is a significant cause of meningitis in many parts of the world, despite available vaccines for some (but not all) meningococcal serogroups. The long-term goal of this project is to understand the role of outer membrane proteins of the meningococcus in pathogenesis of infection, and in eliciting immunity to meningococcal disease. The specific aims will focus on the Class 5 and H.8 proteins, addressing questions on the antigenic structure of the proteins, the genetic mechanisms regulating their synthesis, and their possible role in determining virulence properties of the organism. The general strategy is to use molecular genetic approaches to study the proteins themselves, and to use the mouse model for meningococcal infection to study the role of the proteins in virulence. Class 5 proteins provide an example of the variability of the meningococcal surface. The expression of this family of proteins demonstrates both phase and antigenic variation. Variable and conserved antigenic determinants on the proteins will be mapped by analysis of regions of cloned genes encoding MAb-binding epitopes. Information from the epitope mapping will be used in an analysis of a possible recombinational mechanism that could contribute to structural and antigenic diversity of the proteins. To determine if such recombinational reassortment occurs in natural populations of meningococci, Class 5 genes and epitopes will be analyzed in strains from a single clone isolated from an epidemic in the Gambia. These experiments will also give an indication of the actual extent of variation in Class 5 proteins in an epidemic clone. Finally, the role of Class 5 proteins in virulence will be tested with the adult mouse model of infection, using a modification of the model in which mice remain bacteremic for many days. The H.8 outer membrane protein was identified as one common to all meningococci and gonococci by binding of an H.8-specific monoclonal antibody. The characteristics of the meningococcal H.8 protein will be studied by cloning and sequencing the gene, and by addressing questions on the localization, quantity, and function of the protein. Using the cloned gene, and meningococcal mutant lacking the H.8 protein will be constructed, and the effect of the mutation on virulence in the mouse model will be determined. The mouse model will also be used to determine if immunization with H.8 protein protects against challenge with virulent meningococci.